1. Field of the Invention
The present invention relates to a gear, and image forming apparatuses such as an electrostatic copying machine, a printer, a facsimile machine, and the like provided with the gear.
2. Description of the Related Art
Japanese Patent No. 3666978 discloses a gear including: an outer peripheral portion having a cylindrical shape and provided with teeth; a rotation support portion having a cylindrical shape; an annular rib provided between the outer peripheral portion and the rotation support portion; an arm connecting the rotation support portion and the annular rib; an arm connecting the annular rib and the outer peripheral portion; first radial ribs connecting the rotation support portion and the annular rib; and second radial ribs connecting the annular rib and the outer peripheral portion. The arm connecting the rotation support portion and the annular rib is named as “first arm”, and the arm connecting the annular rib and the outer peripheral portion is named as “second arm”.
In the gear disclosed in Japanese Patent No. 3666978, each of the first and second arms is so formed as to have a plate-like shape which is perpendicular to a center of the gear, and a thickness in an axial direction of each arm is constant. However, an axial thickness of the second arm is thinner than an axial thickness of the first arm. Further, a circumferential thickness of each of the first and second radial ribs is constant. However, a circumferential thickness of the each second radial rib is thinner than a circumferential thickness of each first radial rib.
In such configuration, when a radial distance between the annular rib and the cylindrical outer peripheral portion is short, radial lengths of the second arm and the radial ribs become short. Accordingly, it is possible to improve a strength of the gear while maintaining a teeth surface accuracy i.e. a gear accuracy. However, when the radial distance between the annular rib and the cylindrical outer peripheral portion is long, radial lengths of the second arm and the second radial ribs become long. Accordingly, it is likely that a support strength of the teeth, which is strength of a gear in extended meaning, may be lacked. Further, in this case, repletion of synthetic resin to a forming die at a time of performing an injection molding to form the gear with synthetic resin may become worse, so that a defective molding may occur and a yield rate may be lowered. Further, in an image forming apparatus using such gear lacking strength for driving a rotating member, vibrations due to the defect in meshing of gears and unevenness in rotation may occur, so that an image quality may be impaired. Such problem may occur in a gear having neither an annular rib nor a radial rib.